Northern Arizona lies on the Colorado Plateau, an area of dry plains more than 4,000 ft (1,220 m) high, with deep canyons, including the famous Grand Canyon carved by the Colorado River. Along the Little Colorado River, which runs northwest through the plateau to join the Colorado, are the Painted Desert, where erosion has left colorful layers of sediment exposed, and the Petrified Forest National Park, one of the world's most extensive areas of petrified wood. South of the Grand Canyon are the San Francisco Peaks, including Humphreys Peak, the highest point (12,655 ft/3,857 m) in the state. The southern edge of the Colorado Plateau is marked by an escarpment called Mogollon Rim.
The southern half of the state has desert basins broken up by mountains with rocky peaks and extending NW to SE across central Arizona. To the south, the Gila River, a major tributary of the Colorado, flows west across the entire state. This area has desert plains separated by mountain chains running north and south; in the west the plains fall to the relatively low altitude of c.140 ft (43 m) in the region around Yuma.
Although some mountain peaks receive an annual rainfall of more than 30 in. (76 cm), precipitation in most of the state is low, and much of Arizona's history has been shaped by the inadequate water supply. Since the early 20th cent., massive irrigation projects have been built in Arizona's valleys. Roosevelt, Horse Mesa, Mormon Flat, and Stewart Mountain dams, with reservoirs and storage lakes, irrigate the Salt River valley. The Gillespie Dam on the Gila River helps irrigate the Yuma vicinity. The Coolidge Dam, with its San Carlos reservoir, serves the area near Casa Grande in the southeast. W Arizona is irrigated by Colorado River dams, which also serve California. These include Hoover, Glen Canyon, Davis, Parker, Imperial, and Laguna dams. At the Parker dam, the Central Arizona Project diverts water via canal to Phoenix, the state's capital and largest city, and Tucson, the second largest city. Arizona also obtains water from groundwater pumping stations.
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